Start unit tests for mounting

tests/test_widget_mounting.py

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+import pytest
+
+from textual.app import App
+from textual.widgets import Static
+
+async def test_mount_via_app() -> None:
+    """Perform mount tests via the app."""
+
+    # Make a background set of widgets.
+    widgets = [Static(id=f"starter-{n}") for n in range( 10 )]
+
+    async with App().run_test() as pilot:
+        # Mount the first one and make sure it's there.
+        await pilot.app.mount(widgets[0])
+        assert len(pilot.app.screen.children) == 1
+        assert pilot.app.screen.children[0] == widgets[0]
+
+        # Mount the next 2 widgets via mount.
+        await pilot.app.mount(*widgets[1:3])
+        assert list(pilot.app.screen.children) == widgets[0:3]
+
+        # Finally mount the rest of the widgets via mount_all.
+        await pilot.app.mount_all(widgets[3:])
+        assert list(pilot.app.screen.children) == widgets
+
+    async with App().run_test() as pilot:
+        # Mount a widget before -1, which is "before the end".
+        penultimate = Static(id="penultimate")
+        await pilot.app.mount_all(widgets)
+        await pilot.app.mount(penultimate, before=-1)
+        assert pilot.app.screen.children[-2] == penultimate
+
+    async with App().run_test() as pilot:
+        # Mount a widget after -1, which is "at the end".
+        ultimate = Static(id="ultimate")
+        await pilot.app.mount_all(widgets)
+        await pilot.app.mount(ultimate, after=-1)
+        assert pilot.app.screen.children[-1] == ultimate
+
+    async with App().run_test() as pilot:
+        # Mount a widget before 0, which is "at the start".
+        start = Static(id="start")
+        await pilot.app.mount_all(widgets)
+        await pilot.app.mount(start, before=0)
+        assert pilot.app.screen.children[0] == start
+
+    async with App().run_test() as pilot:
+        # Mount a widget after 0. You get the idea...
+        second = Static(id="second")
+        await pilot.app.mount_all(widgets)
+        await pilot.app.mount(second, after=0)
+        assert pilot.app.screen.children[1] == second
+
+    async with App().run_test() as pilot:
+        # Mount a widget relative to another via query.
+        queue_jumper = Static(id="queue-jumper")
+        await pilot.app.mount_all(widgets)
+        await pilot.app.mount(queue_jumper, after="#starter-5")
+        assert pilot.app.screen.children[6] == queue_jumper
+
+    async with App().run_test() as pilot:
+        # Mount a widget relative to another via query.
+        queue_jumper = Static(id="queue-jumper")
+        await pilot.app.mount_all(widgets)
+        await pilot.app.mount(queue_jumper, after=widgets[5])
+        assert pilot.app.screen.children[6] == queue_jumper
+
+    async with App().run_test() as pilot:
+        # Make sure we get told off for trying to before and after.
+        await pilot.app.mount_all(widgets)
+        with pytest.raises(Static.MountError):
+            await pilot.app.mount(Static(), before=2, after=2)
+
+    async with App().run_test() as pilot:
+        # Make sure we get told off trying to mount relative to something
+        # that isn't actually in the DOM.
+        await pilot.app.mount_all(widgets)
+        with pytest.raises(Static.MountError):
+            await pilot.app.mount(Static(), before=Static())
+        with pytest.raises(Static.MountError):
+            await pilot.app.mount(Static(), after=Static())